2.1. The Role of Surfactant in Pulmonary Physiology.
Inhaled air containing oxygen travels through the trachea, the bronchi, and the bronchioles to the hundreds of millions of terminal alveoli. The terminal alveoli are the air spaces in the lungs where oxygen is taken up by the blood in exchange for carbon dioxide.
At the interface between the gas in the terminal alveoli and the liquid of the lung tissue, (i) oxygen diffuses into the blood from the alveoli and (ii) carbon dioxide diffuses from the blood to the alveolar air before being exhaled. To diffuse from the alveolar gas to the blood, an oxygen molecule must traverse the liquid lining the alveoli, at least one epithelial cell, the basement membrane, and at least one endothelial cell.
In order to attain sufficient uptake of oxygen by the blood and excretion of carbon dioxide from the blood, an animal's lungs must ventilate the terminal alveoli simultaneously and evenly. Either unsynchronized or uneven ventilation will prevent sufficient oxygen uptake into the circulating blood and result in the retention of carbon dioxide in the body.
Pulmonary surfactant acts at the interface between alveolar gas and the liquid film lining the luminal surface of the cells of the terminal alveoli. The normal pulmonary surfactant lining is extremely thin, usually no more than 50 nm thick. Thus, the total fluid layer covering the 70 square meters of alveolar surface in an adult human is only 35 ml.
For materials to be effective lung surfactants, surfactant molecules must move rapidly to the surface of the liquid. Pulmonary surfactant functions by adsorbing to the surface of the liquid covering these lining cells and changing surface tension of the alveolar fluid during the respiratory cycle.
Surface tension is a characteristic of most liquid solutions. At the interface between liquid and a gas phase, the movement of molecules at the surface of the liquid is restricted by intermolecular forces acting on those molecules. The intermolecular forces have a net direction that tends to decrease the area of the surface. The net force at the surface is referred to as surface tension. Surface tension varies with molarity, temperature and multiple solutes. Surface tension has units of force per unit length (dynes/cm or mN/m). The vector of the surface tension force is perpendicular to the plane of the interface.
The lungs of vertebrates contain surfactant, a complex mixture of lipids and protein which causes surface tensions to rise during surface expansion (inflation) and decrease during surface compression (deflation). During lung deflation, surfactant decreases surface tension to near .ltoreq.1 mN/m, so that there are no surface forces that would otherwise promote alveolar collapse. Aerated alveoli that have not collapsed during expiration permit continuous O.sub.2 and CO.sub.2 transport between blood and alveolar gas and require much less force to inflate during the subsequent inspiration. During inflation, lung surfactant increases surface tension as the alveolar surface area increases. A rising surface tension in expanding alveoli opposes over-inflation in those airspaces and tends to divert inspired air to less well-aerated alveoli, thereby facilitating even lung aeration.